The present invention relates to a process for preparing glass ceramic green sheets for use in the production of substrates in glass ceramic printed circuit boards on which electronic devices such as LSI's are mounted.
In recent years, plastic substrates, such as those made of a phenolic resin or glass-reinforced epoxy resin, which have predominantly been used as substrates in conventional printed circuit boards, have rapidly been replaced by ceramic substrates, particularly ceramic multilayer substrates due to the demand for increased mounting density and improved reliability of printed circuit boards.
Ceramic multilayer substrates are usually produced either by the multilayer printing process or the green sheet laminating process. In the multilayer printing process, a conductive paste and an insulating paste are alternately applied in predetermined patterns onto a sintered or green ceramic sheet, which is then fired. The green sheet laminating process comprises laminating a plurality of green sheets, each having a desired circuit pattern printed thereon with a conductive paste and through holes (also called via holes) filled with a conductive paste, and co-firing the resulting laminate to sinter the green sheets and the conductive paste simultaneously. Of these, the green sheet laminating process is prevalent because a larger number of layers can be readily laminated with more precise circuit patterns.
As a ceramic material for ceramic substrates, alumina (Al.sub.2 O.sub.3) has primarily been employed in view of its good electrical insulating properties and heat resistance and its relatively low material costs. Since alumina-based ceramics are sintered at a temperature as high as 1550.degree. C., it is necessary to use a conductive paste containing a conductive powder of a refracfory metal, e.g, W (tungsten) or Mo (molybdenum) or a mixture thereof, to print alumina-based green sheets or fill through holes formed therein such that the conductive powder withstands the high temperature during firing without a significant loss of conductivity.
However, the electric resistivity of W and Mo is relatively high for a metal. Therefore, as circuits become finer and thinner to increase the mounting density, the resistance of circuits made of W or Mo is appreciably increased to such a degree that it may interfere with the function of the circuit by an increased signal delay.
In order to eliminate the problem just described which has been encountered by the use of W or Mo powder in a conductive paste, multilayer substrates of low-firing ceramic materials have been developed. These ceramic materials can be sintered at a temperature below 1000.degree. C., and therefore it is possible to fire them along with a non-refractory, low-resistivity conductive metal such as Ag or Cu in a conductive paste to form substrates.
One class of low-firing ceramic substrates is a glass ceramic substrate. Glass ceramic substrates are attracting much attention for the reason that they have insulating properties and heat resistance comparable to conventional alumina substrates, a dielectric constant lower than alumina (leading to a reduced signal delay) and a thermal expansion coefficient close to that of silicon (thereby facilitating mounting of flip chips). The raw material for typical glass ceramic substrates is a combination of a boron-containing glass powder such as a powder of a borosilicate-based glass, MgO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass, or CaO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass, with a ceramic powder such as an alumina powder which serves as a filler.
Glass ceramic multilayer substrates are generally prepared as follows. A glass powder and a ceramic powder are subjected to wet grinding together in a ball mill until the particle size is reduced to a level suitable for use in tape casting into green sheets (e.g., 1 to 5 .mu.m in average particle diameter), thereby achieving Both size reduction and thorough mixing of the glass and ceramic powders. Commercially available glass powders and ceramic powders are normally coarse and have an average particle diameter in the range of about 5 .mu.m to about 100 .mu.m. Therefore, it is necessary to reduce the particle size of the powders in the first step. The size reduction or comminution has normally been performed by wet grinding in a ball mill using water as a liquid medium.
Subsequently, the ground powder mixture is recovered and dried to remove water. In most cases the dried powder mixture is agglomerated. Therefore, the powder mixture is disintegrated before it is mixed with an organic solvent as a dispersing medium, an organic binder, and other additives such as a dispersant and plasticizer to form a slurry called a "slip". The slurry is then cast into sheets by a suitable method, typically by the tape casting method using a doctor blade, and glass ceramic green sheets are obtained after the sheets are dried to remove most of the solvent.
The resulting glass ceramic green sheets are punched to form through holes, if necessary, and a conductive paste is then printed onto each sheet to form a circuit pattern thereon and fill the through holes, if present. A plurality of such green sheets are laminated, and the resulting laminate is co-fired at a temperature below 1000.degree. C. (usually between 900.degree. C. and 1000.degree. C.) to sinter the conductive paste and green sheets simultaneously to give a glass ceramic multilayer substrate. The firing step is usually preceded by a degreasing step whereby organics are removed from the laminate by heating to a temperature substantially lower than the firing temperature.
In the preparation of alumina-based ceramic green sheets which are fired at a higher temperature, it is known that a fine alumina powder which has been comminuted is processed in a wet ball mill, for purposes of disintegration and mixing with additives such as an organic binder, using an organic solvent as a liquid medium. See, Japanese Patent Application Kokai No. 59-195573(1984). However, such wet ball milling is not intended for comminution or substantial size reduction of the alumina powder. Whether the green sheets to be prepared are of a low-firing glass ceramic or a conventional high-firing ceramic such as alumina, wet ball milling in water has been employed in the grinding step to reduce the particle size of a raw material to a level suitable for tape casting.
In order to meet the recent demand for still higher integration of LSI's and further size reduction of printed circuit boards, through holes formed in green sheets are required to have a smaller diameter and a smaller pitch.
In general, when a ceramic green sheet is punched to form through holes with a smaller diameter and pitch, it is important to prevent as much as possible the formation of burrs around each hole and the formation of cracks between adjacent holes. For this purpose, it is advantageous that the ceramic green sheet have an increased elongation.
A ceramic green sheet is assured to have an increased elongation if the surfaces of the ceramic powder particles present in the sheet are completely covered with an organic binder, thereby improving the adhesion between ceramic powder particles.
In the prior art glass ceramic green sheets prepared in the above-described manner, however, it was frequently found that an increased elongation could not be achieved due to failure to completely cover the surfaces of the powder particles unless an excessively large amount of the organic binder is added. The use of the organic binder in a significantly increased amount not only adds to the manufacturing costs of the green sheets but also may increase the amount of residual carbon remaining after firing, which causes the resulting sintered substrate to turn gray and have a decreased insulation resistance.